Report of group 1: Luca Ilic, Alex Myagkov, Ioannis Binietoglou, Karen Acheson, Jordi Tiana-Alsina, Nils Küchler


Water vapour (WV) is an effective greenhouse gas and therefore important for the earth’s energy budget. Furthermore, condensation and convection depend on the relative humidity of the atmosphere. In order to determine the possibility of convection and formation of clouds that influence the radiation budget and to provide humidity information to weather forecast models it is necessary to measure the distribution of WV with high resolution in space and time. Direct and indirect (both passive and active) methods are used to obtain information about the WV concentration.

During the 1st ITaRS summer school and field campaign in Bucharest, the products of the above introduced instruments have been evaluated with respect to resolution and accuracy.

Microwave radiometers measure emitted radiation along the pressure broadened WV emission line at 22.235 GHz and provide vertical humidity profiles up to 10 km altitude. The vertical extension of radiosoundings is even higher and the relative humidity is determined by direct measurements. Moreover sun photometers derive the integrated water vapour comparing the optical depths at three frequencies. A further opportunity is to use the WV Raman Lidar detecting the Raman transition of WV.

Raman lidar can be used to monitor the quantity of water vapour in the atmosphere by measuring the inelastic Raman scattering of laser light by water vapour molecules. Based on these measurements, the water vapour mixing ratio can be retrieved from near the ground up to several kilometres with high vertical and temporal resolution. The limitation of this method is that it requires calibration based on other reference measurements.


A 15 day time series of integrated water vapour data from microwave radiometer, radiosoundings and photometer was taken between 15-30 September. Good agreement was seen between the values over this time range. Two techniques were used to calibrate the Raman Lidar. Firstly, the WV mixing ratio was calculated with the radiosounding measurements. Secondly, the lidar profile was calibrated against the integrated water vapour values from the microwave radiometer (Figure 1).

Group 1 image 1

Figure 1: Calibration comparison of microwave radiometer and radiosonde, 28th of September, 00 UTC. Location Soundings, Bucharest Aiport. Location MWR: Magurele.

A good agreement was found between the two calibration techniques with a discrepancy of <10%. Collocated and simultaneous measurements have been found to be necessary to retrieve reliable calibration values.